1. Field of the Invention
The present inventions relate to an inspection system for glass sheet material and, more particularly, consecutive inspection for defects in coated and non-coated glass.
2. Description of the Related Art
During the manufacture of transparent materials, optical defects and deviations may be produced that render the transparent material imperfect. Optical imperfections are of special concern in glass and plastic sheet applications where optical defects are unacceptable from a quality control standpoint. Optical quality defects include surface imperfections, inclusions and inconsistencies in light transmission or reflection. Scratches, digs, coating anomalies, seeds, stones, chips, particles, tin residue, ripples, distortion, and grind lines are common categories of optical quality defects.
Insulated glass (IG) units used in the manufacture of windows are made of two sheets of glass sealed together with a spacer between the two sheets of glass. One sheet of glass is often coated with a thin-film low emissivity (low-E) coating prior to assembly of the IG unit. The low-E coating improves the insulating value of the window by reflecting much of the infrared (IR) and heat-carrying portion of the spectrum. Windows manufactured with low-E coatings typically cost about 10-15% more than IG units made with uncoated glass, but they reduce energy loss through the window by as much as 30-50%.
IG units are manufactured in many ways, ranging from manual assembly to highly automated assembly with little or no human handling. Automated IG unit assembly involves loading two matching glass sheets in sequence onto an assembly line. A typical IG unit consists of one clear sheet of glass and one low-E coated sheet of glass, although other variations are possible such as two clear sheets of glass. After the two glass sheets are loaded onto the assembly line, the low-E coated glass sheet is “edge-deleted” whereby about 0.375 inch of the coating is removed from the perimeter of the glass sheet by grinding. Removal of the coating is necessary to avoid corrosion of the low-E coated glass after assembly into an IG unit. Edge deletion is described in more detail in US Patent Application No. 2003/0024180 A1. The proper geometry for the edge deletion is typically a uniform band measuring 0.375 inches in width from the edge of the glass. After the low-E glass has the coating removed from the edge, the glass sheets are washed, inspected, a spacer with sealing material is applied, the unit is pressed together, trapped air may be removed and a low-conductivity gas such as Argon may be added, and a secondary seal is applied.
Optical inspection of the glass sheets prior to assembly into an IG unit is highly desirable. Once a defective sheet of glass is assembled into an IG unit, the entire IG unit is deemed defective and must be discarded. An IG unit is valued at approximately ten times the value of a single glass sheet. Optical inspection by humans is inconsistent, subject to high error rates and expensive. Automation of the optical inspection process is highly desirable.
A fundamental problem with automated optical inspection relates to the variance in the optical properties of clear glass as compared to coated glass. The application of multi-layer, optical thin-films coatings to glass results in changes in color, reflection and transmission of light as compared to clear glass. A wide variety of thin film coatings are produced for the window industry, including many types of low-E coatings. The various coatings offer variety in color, reflection, transmission and solar control properties. The proper and accurate optical inspection of glass sheets necessitates unique configuration of the automated optical inspection system appropriate for the type of glass and coating being inspected. Unique configurations include specific lighting arrangements, camera settings such as aperture and shutter speed, and unique inspection variables within the logic governing the automated inspection.
The optical quality of flat glass used in IG units, can be determined through the use a digital imaging device arranged to inspect the glass. This inspection could be done using illumination that reflects off the glass or transmits through the glass. The focus of the digital imaging device is on the glass sheet and the camera generates signals revealing the quality of the glass.
The use of artificial vision for the automated inspection of transparent materials and glass in particular is currently limited by a lack of effectiveness for the detection of imperfections in and on coated and uncoated glass on the same production line.